1. Field of the Invention
The invention relates generally to a method of generating electrical power using an advanced thermochemical recuperation cycle and, more specifically, to a method of increasing the ratio of power generated by a gas turbine to that generated by a steam turbine, and to improving the net power generation efficiency relative to a conventional combined-cycle power plant.
2. Brief Description of Related Technology
The combustion of gaseous fuels is a key characteristic of most modern electrical power generation processes. Overall performance of each process is often measured by the efficiency at which energy may be collected from a given amount of fuel burned in the process. Sensible heat present in the combustion exhaust gas is used to generate pressurized steam to drive one or more rotating turbines coupled to electric generators to produce electricity. The thermodynamic cycle in which water is converted to pressurized steam in a water boiler, the pressurized steam is passed through a turbine having rotating blades where the steam expands, and thereafter is condensed before being pumped back to the water boiler, is known as the Rankine cycle. Because of its relative simplicity and reliability, power plants using one or more Rankine cycles have dominated the electrical power generating industry. Numerous modifications have been attempted to optimize the efficiency of Rankine cycles such as, for example, the use of high-pressure, intermediate-pressure, and low-pressure turbines, which together permits more efficient power generation as high-pressure steam is expanded in a stage-wise manner. See generally, Collins, S. "Power Generation." in: Encyclopedia of Chemical Technology (1996 ed.), vol. 20, pp. 1-40. Ref. TP9.E685.
In a conventional simple-cycle power plant, compressed air and a fuel are fed to a gas-fired combustor where the fuel is burned in the presence of oxygen-containing gas (usually air) to produce a hot exhaust gas. The hot exhaust gas is then fed to one or more gas turbines coupled to electric generators where the gas expands (and cools), thereby causing turbine blades to rotate, producing mechanical energy which is converted to electricity in the generators. The expanded exhaust gas having passed through the turbines may, optionally, undergo various unit operations where toxins and other pollutants may be removed. The cooled exhaust gas eventually is vented to the atmosphere. Modern simple-cycle power plants are able to obtain an overall energy efficiency of about 35% to about 38% (calculated as the energy equivalent electricity produced relative to the lower heating value (LHV) of the fuel fed to the combustor, referred to hereinafter as the "LHV basis").
Combined-cycle power plants improve on the efficiency of the simple-cycle power plant by using the sensible heat remaining in the hot exhaust gas of a simple-cycle power plant to drive another power cycle, typically the Rankine cycle--hence the name "combined-cycle power plant." Modern combined-cycle power plants typically are capable of an overall energy efficiency of about 45% to about 55% (LHV basis). Thus, use of a Rankine cycle in combination with a simple-cycle plant results in approximately a 10% to a 17 point increase in efficiency over the simple-cycle plant alone. In fact, bottoming steam cycles can account for about 30% to about 40% of the overall power output of a combined-cycle power plant.
For a number of reasons, however, it is not desirable to have a bottoming steam cycle that generates a large share of the total power. One reason is that steam cycles are very difficult to control precisely. Power generated by steam cycles is controlled ultimately by the amount of gaseous fuel burned in the combustors and the amount of heat remaining in the hot turbine exhaust gas. When it is desired to increase power generation (e.g., during peak load periods) or decrease power generation (e.g., during partial load periods) to match power consumption demands, the most practical way to change the power output is to increase or reduce the flow of gaseous fuel fed to the combustors. In so doing, the amount of power generated by both of the gas turbines and the steam turbines would be affected. Since each of the gas turbines and the steam turbines produce large shares of the total power generated by the plant, careful selection of gaseous fuel flowrates must be made. This proves to be difficult to accomplish even with the advent of complex process control computers. The difficulty is exacerbated by the use of multi-pressure steam turbines in the bottoming steam cycle. Hence, switching between partial and peak load operation of conventional combined-cycle power plants poses many problems not effectively addressed by the prior art.
Additionally, steam turbines generally are less efficient than gas turbines and are more costly to design and operate, as are the corresponding heat exchange equipment. Therefore, it would be desirable to generate a higher percentage of the overall power using the upstream gas turbines, and a lower percentage of the power in the steam turbines. Despite advances in the machinery (e.g., turbines, compressors, combustors, etc.), there are limits on the power generation efficiency of conventional combined-cycle power plants.
Prior art attempts to increase the power generation efficiency of the gas turbines include the use of thermochemical recuperation (hereinafter "TCR") cycles. One particular type of TCR cycle employs sensible heat in the hot exhaust gas of a gas turbine to supply the heat necessary for endothermic catalytic conversion of hydrocarbons (e.g., natural gas or derivatives thereof) into a desirable combustible fuel which then may be combined with compressed air and burned in the combustors upstream of the gas turbines. More specifically, in this TCR cycle, heat is transferred in a chemical reactor (recuperator) from the exhaust gas of a gas turbine to a reacting natural gas/steam mixture which is passed over a steam-reforming catalyst (e.g., a nickel-based catalyst) at high temperatures and is converted to a desirable combustible mixture of hydrogen and carbon monoxide. In such a recuperator, heat is absorbed by the combustible mixture and is released in the subsequent combustion of the mixture in the combustor. An example of this type of TCR cycle is disclosed in EP 761,942 (hereinafter "EP '942") and has a stated overall energy efficiency of about 48% (LHV basis), which, according to EP '942 is about "20% higher than what is otherwise obtained in a conventional simple-cycle plant." (See column 3, lines 48-51 of EP '942.)
In a conceptually different TCR cycle, such as the one disclosed in Harvey, S.P., et al., Reduction of Combustion Irreversibility in a Gas Turbine Power Plant Through Off-Gas Recycling, Journal of Engineering for Gas Turbines and Power, Vol. 117, no. 1 (Jan. 1995), pp. 24-30 (hereinafter the "Harvey et al. publication"), the exhaust gas of a gas turbine is used to directly and indirectly heat a mixture of raw fuel, water, and a part of a recycled exhaust gas (following cooling and water condensation), into a reformed fuel fed to the combustor. More particularly, the heat recovered from the turbine exhaust gas is used to drive an endothermic reforming reaction in which the raw fuel is heated and partially oxidized in a thermochemical recuperator with oxygen-containing constituents (i.e., carbon dioxide and water) present in the recycled exhaust gas. The reformed fuel and most of the recycled exhaust gas (comprising nitrogen, carbon dioxide, and other inert gas) are then burned stage-wise in the presence of air in three combustors, each of which is associated with a gas turbine. An exhaust gas exits the combustors and is passed through the turbines, followed by partial combustion with added air to result in a purported power generation efficiency of up to 65.4% (LHV basis).
There are several problems associated with the particular TCR cycle described in the foregoing paragraph and the publication cited therein. First, a significant amount of environmentally harmful nitrogen oxides (generally designated "NO.sub.x ") are undesirably formed during the combustion of the reformed fuel and can undesirably accumulate in the recycled exhaust gas/water streams. The accumulation of NO.sub.x can cause severe corrosion problems in recycle gas compressors and related process equipment.
Second, temperatures of about 1382.degree. F. (about 750.degree. C.) to about 1700.degree. F. (about 927.degree. C.) are required to appreciably reform a natural gas (containing mostly methane gas) with recycled steam and carbon dioxide. For a TCR application using turbine exhaust gas as the heat source for the endothermic reforming reaction, this would require significantly high turbine exhaust gas temperatures. According to FIG. 1 of the Harvey et al. publication, such high temperatures are ensured because the combustion exhaust gas exits the combustor and enters the three-stage gas turbine at a temperature of 2300.degree. F. (about 1260.degree. C.) and, thereafter, passes through and exits the third-stage gas turbines at a temperature of 1821 .degree. F. (about 994.degree. C.). It is known that conventional gas turbines are designed to operate efficiently within a temperature range of about 1050.degree. F. (about 565.degree. C.) to about 1150.degree. F. (about 620.degree. C.). The lower exhaust gas temperatures enable conventional turbines to extract more work, i.e., generate more electrical power, per turbine. Because the temperature of the exhaust gas according to the design disclosed in the Harvey et al. publication is so high, it teaches the use of staged gas turbines to achieve a similar work load. The Harvey et al. publication at page 26, discloses state-of-the-art turbines, such as high-temperature metal turbines employing circulating cooling air and other alternatives for high-temperature gas turbine operation. These alternative turbines, however, are very expensive. Additionally, the use of staged expansion requires the use of multiple turbines in series, which is expensive.
Generally, it is known that TCR cycles can be used to boost power generating efficiency of simple-cycle power plants. There is no teaching in the prior art of using a bottoming Rankine cycle in combination with a TCR cycle. However, even if the success of Rankine cycles in combined-cycle power plants would lead one of ordinary skill in the art to conclude that such a bottoming Rankine cycle could be useful in combination with a TCR cycle, the problems associated with the use of steam turbines in the Rankine cycle and the fact that the Rankine cycle will generate too high a share of the overall power generated by the power plant are problems not resolved by the previously known systems. Furthermore, one of ordinary skill in the art would not conclude that such TCR cycles, in combination with a bottoming Rankine cycle could match or improve on the power generating efficiency of conventional combined-cycle power plants.
The overall efficiency of a power plant is a function of the efficiency of the gas turbine(s). Gas turbine efficiency, in turn, is a function of combustor air and fuel inlet temperatures, turbine inlet temperature, and upstream compressor efficiencies, to name a few. Certain of these factors are determined by the process equipment and/or the type of power cycle(s) employed, while other factors are not readily controllable. Overall efficiency, however, is but one characteristic of a power plant design. The ability to safely and easily accommodate peak and partial load power demands is another characteristic of the design. Heretofore, there has not been a suitable design which improved on both of these characteristics, especially the characteristic of peak and partial load operation, thereby providing the ability to safely and easily accommodate power plant operation during periods of partial and peak load demands.